Ingenious techniques are needed to extend group IV photonics from near-infrared to mid-infrared wavelengths. If achieved, the reward could be on-chip CMOS optoelectronic systems for use in spectroscopy, chemical and biological sensing, and free-space communications.

Mid-infrared photonics in silicon and germanium

Silicon lasers have long been a goal for semiconductor scientists, and a number of important breakthroughs in the past decade have focused attention on silicon as a photonic platform. Here we review the most recent progress in this field, including low-threshold silicon Raman lasers with racetrack ring resonator cavities, the first germanium-on-silicon lasers operating at room temperature, and hybrid silicon microring and microdisk lasers. The fundamentals of carrier transition physics in crystalline silicon are discussed briefly. The basics of several important approaches for creating lasers on silicon are explained, and the challenges and opportunities associated with these approaches are discussed. Silicon lasers have long been a goal for semiconductor scientists. This Progress Article reviews the most recent developments in this field, including silicon Raman lasers, the first germanium-on-silicon lasers operating at room temperature, and hybrid silicon microring and microdisk lasers. Challenges and opportunities for the present approaches are also discussed.

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Recent progress in lasers on silicon

Lasing is experimentally demonstrated in a direct bandgap GeSn alloy, grown directly onto Si(001). The authors observe a clear lasing threshold as well as linewidth narrowing at low temperatures.

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Lasing in direct-bandgap GeSn alloy grown on Si

Electrically pumped lasing from Germanium-on-Silicon pnn heterojunction diode structures is demonstrated. Room temperature multimode laser with 1mW output power is measured. Phosphorous doping in Germanium at a concentration over 4x1019cm-3 is achieved. A Germanium gain spectrum of nearly 200nm is observed.

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An electrically pumped germanium laser

Reliable, efficient electrically pumped silicon-based lasers would enable full integration of photonic and electronic circuits, but have previously only been realized by wafer bonding. Here, we demonstrate continuous-wave InAs/GaAs quantum dot lasers directly grown on silicon substrates with a low threshold current density of 62.5 A cm–2, a room-temperature output power exceeding 105 mW and operation up to 120 °C. Over 3,100 h of continuous-wave operating data have been collected, giving an extrapolated mean time to failure of over 100,158 h. The realization of high-performance quantum dot lasers on silicon is due to the achievement of a low density of threading dislocations on the order of 105 cm−2 in the III–V epilayers by combining a nucleation layer and dislocation filter layers with in situ thermal annealing. These results are a major advance towards reliable and cost-effective silicon-based photonic–electronic integration.

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Electrically pumped continuous-wave III–V quantum dot lasers on silicon

Monolithic lasers on Si are ideal for high-volume and large-scale electronic-photonic integration. Ge is an interesting candidate owing to its pseudodirect gap properties and compatibility with Si complementary metal oxide semiconductor technology. Recently we have demonstrated room-temperature photoluminescence, electroluminescence, and optical gain from the direct gap transition of band-engineered Ge-on-Si using tensile strain and n-type doping. Here we report what we believe to be the first experimental observation of lasing from the direct gap transition of Ge-on-Si at room temperature using an edge-emitting waveguide device. The emission exhibited a gain spectrum of 1590-1610 nm, line narrowing and polarization evolution from a mixed TE/TM to predominantly TE with increasing gain, and a clear threshold behavior.

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Ge-on-Si laser operating at room temperature.

Ge-on-Si optoelectronics

Direct band gap narrowing in highly doped n-type Ge is observed through photoluminescence measurements by determining the spectrum peak shift. A linear relationship between the direct band gap emission and carrier concentration is observed. We propose a first order phenomenological model for band gap narrowing based on two parameters whose values for Ge are EBGN = 0.013 eV and ΔBGN = 10−21 eV/cm−3. The application of these results to non-invasive determination of the active carrier concentration in submicron areas in n-type Ge structures is demonstrated.

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Direct band gap narrowing in highly doped Ge

We demonstrate CVD in situ doping of Ge by utilizing phosphorus delta-doping for the creation of a high dopant diffusion source. Multiple monolayer delta doping creates source phosphorous concentrations above 1 × 1020cm−3, and uniform activated dopant concentrations above 4 × 1019cm−3 in a 600-800nm thick Ge layer after in-diffusion. By controlling dopant out-diffusion, near-complete incorporation of phosphorus diffusion source is shown.

Rodolfo Camacho-Aguilera

Papers

Ge-on-Si laser operating at room temperature.

An electrically pumped germanium laser

Ge-on-Si optoelectronics

Direct band gap narrowing in highly doped Ge

High active carrier concentration in n-type, thin film Ge using delta-doping